High-temperature warning system

ABSTRACT

The present invention provides a novel temperature warning device comprising only a tungsten filament lamp, a thermistor sensor device and a voltage source all in series circuit relation, which system when properly designed (i.e., the thermal time constant of the lamp exceeds that of the sensor) provides a visible signal which is a function of the temperature of the sensor&#39;&#39;s housing. The present invention relates generally to temperature sensor and warning systems and more particularly to systems for determining when the temperature of an environment has exceeded a specified predetermined maximum.

United States Patent The United States of America as represented by theSecretary of the Army [73] Assignee [54] HIGH-TEMPERATURE WARNING SYSTEM2 Claims, 4 Drawing Figs.

[52] US. Cl 340/228 R, 338/25, 340/227 R, 340/331 [51] Int. Cl G08b17/06 [50] Field of Search 340/228 R; 338/25 [56] References CitedUNITED STATES PATENTS 2,106,249 l/l938 Hower 338/25 2,274,830 3/1942Gould et a1. 338/25 3,418,648 12/1968 Futaki 340/228 3,475,745 10/1969White 340/228 X Primary Exnminer-David L. Trafton Attorneys- Harry M.Saragovitz, Edward J Kelly, Herbert Berl and Arthur L. GirardHIGH-TEMPERATURE WARNING SYSTEM DESCRIPTION OF THE DRAWINGS FIG. 1 showsa schematic diagram of the thermistor, tungsten lamp sensor circuit ofthe present invention;

FIG. 2 shows one embodiment of a thermistor suitable for use in thecircuit of FIG. 1;

FIG. 3 shows an alternate preferred embodiment of a thermistor suitablefor use in the circuit of FIG. 1; and

FIG. 4 is a graphic representation of the theory of operation of thethermal switch of FIG. 1.

SUMMARY OF THE INVENTION A series circuit consisting only of a tungstenfilament lamp,

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic diagram of thethermistor, tungsten lamp sensor circuit of the present invention;

FIG. 2 shows one embodiment of a thermistor suitable for use in thecircuit ofFlG. 1;

FIG. 3 shows an alternate preferred embodiment of a thermistor suitablefor use in the circuit of FIG. 1; and

FIG. 4 is a graphic representation of the theory of operation of thethermal switch of FIG. 1.

DETAIL DESCRIPTION OF THE INVENTION Thermistors have a condition calledthermal runaway which occurs due to the self-dissipation of suchdevices. In a typical thermistor there are two extremes of thermalenvironment in which the thermistor can be considered. These two are anadiabatic system and an isothermal systemrln an isothermal system, theresistance of the thermistor is dependent only on the temperature of thetotal isothermal system. In an adiabatic system however, the temperatureof the thermistor will, if any current is flowing through it, climbuntil the device either destroys itself or reaches a condition of zeroohms, i.e., no further dissipation is produced. The theory of theinstant temperature warning circuit utilizes a combination of these twoextremes.

By controlling the thermal conductivity of the container in which thethermistor is mounted by utilizing a thermal resistance coating of aninsulating material, and immersing this container in a fluid whosetemperature one wishes to sense (thus the thermistor device becomes thesensing element in the system), the thermistor is made to switch (in acontrolled thermal runaway) at a specific temperature.

If the above described thermistor is placed in a circuit with a deviceexternal to the sensing element which has a resistance changecharacteristic which is opposite of that of the thermistor (athermistors resistance decreases with increasing temperature, therefore,the external device must have resistance increase with increasingtemperature) then another phenomenon occurs. The external device, afterthe thermistor switches, i.e., undergoes controlled thermal runaway,decreases the power dissipation in the thermistor by heating andincreasing its resistance thereby switching the thermistor back to itsinitial condition. Refractory materials such as tungsten, provide thissecond external device characteristic.

' Therefore, by the proper design of a thermistor and a tungstenfilament lamp, one can make a device which will cycle between the twostates when the thermistorvmounting fixture is at or above a specifictemperature.

In order to provide cyclic operation one additional important conditionmust be met. That condition is that the thermal time constant (i.e., theperiod of time required for the lamp to heat up to the switchingtemperature or cool down and therefore become lower in resistance to thepoint where the thermistor can undergo runaway" once again) of the lampmust be longer than the thermal time constant of the thermistor (i.e.,the period of time required for the thermistor to reach a condition ofzero ohms or self-destruction) to preclude the possibility of the systemcoming to rest at an equilibrium point halfway through the switchingoperation.

On a servo or closed loop basis, the operational characteristics of thethermistor can be considered as positive feedback and the lamp asnegative feedback. The basic circuit as shown in FIG. 1 is a seriesconnection of a suitable tungsten lamp and thermistor driven by avoltage source. Thus, as shown in FIG. 1 the thermal sensing system ofthe present invention comprises, in series circuit relation, a tungstenfilament lamp l0 and a thermistor l2 driven by an input voltage frompower source 14. j

As already described the lamp for this system must have a long thermaltime constant. Such a constant is provided by making the filament of thelamp much more massive than that in a conventional lamp design. This isaccomplished by using a longer length of heavier wire and closelycoiling it to approach the same cold and hot resistance as a normallamp. In this configuration the thermal mass of the filament is muchhigher than a normal lamp and it takes a much longer time to heat orcool the filament.

When properly designed as described below the system can be made toprovide an indication of proper system functioning by having thetungsten lamp glow dimly at temperatures below the switching temperature(in a safe" temperature condition). At extremely low, decreasing,temperatures, i.e., below .about 32 F., the thennistor resistance isrising rapidly, and

could cause the lamp to become invisible. This phenomenon, although notnecessarily detrimental to the successful use of this system, can beprevented by depositing a resistance film of, for example, nichrome overthe body of the thermistor, which limits the maximum resistance to avalue sufficient to keep the lamp filament of 1,000 F., a temperature atwhich the filament provides a dull red glow similar to thatexperiencedin the filament of vacuum tubes; such a coating will bedescribed below. In the overtemperature or switched" condition, the lamptemperature, i.e., the filament temperature, approaches about 3,200 F.The resistance change in the lamp under these conditions isapproximately 2.5 to 1. Under these conditions the system then functionsin the following manner:

a. normal operation, safe temperature, lamp panel glows dimly;

b. normal operation, overtemperature, flashing of lamp approximately lOtimes per minute depending upon the particular filament and thermistorused,

c. faulty system, open circuit, lamp panel dark;

d. faulty system, due to short circuit of sensor, steady bright light.

The theory of operation of the thermal switch is derived from the graphshown in FIG. 4 and the equation set forth .ll q f j..

Tr BT, RL VDC V out B R. vwt (Wm VDC' Vout Y 1;

R1. VDC' W VIDC wherein:

T,,Ambient temperature'of sensor T,,Reference temperature (25 C.)

BA constant characteristic of the thermistor R,,Resistance of thermistorat T R, Resistance of lamp VDCInput voltage Yqq zYql is 19 RL K-Aconstant defining thermal resistance The first term of the equationdelineates the behavior of the thermistor in an isothermal environment.The second term delineates the effects of self-heating due to internaldissipation and thermal resistance. It should be observed that in thisequation input voltage is expressed as a ratio of input to output anddrops out as a factor in the equation. As should be clear from a studythereof the equation can be multivalued, i.e., having three solutionswhen the second term is of sufficient magnitude. The plot of this curveis shown in FIG. 4, plotted for three values of R R =l unit, R, =l .5units, and R =2 units. At a specific ambient temperature, the lamp has aresistance R If the ambient temperature reaches the switchingtemperature T the thermistor will drop its resistance due to thermalselfheating, and will abruptly raise the voltage across R This highervoltage will cause the lamp to brighten markedly, and the lamp filamentwill heat and increase its resistance. This moves the characteristiccurve through an infinite family of curves until the thermistor cannotdraw sufficient power for self-heating (the function becomes singlevalued) and the lamp power decreases, lowering the resistance until theinitial conditions are reestablished and the entire process repeats.

This results in the blinking of the lamp in an overtemperaturecondition. If the temperature of the ambient decreases slightly belowthe trigger point, the lamp will stop flashing, since it cannot climbover the peak point of the curve to switch on.

A thermistor of the type suitable for use in the present invention isshown in FIG. 2. This comprises bulk thermistor l6 coated on opposingsides thereof with a conductive resistance film 18, the thermistorhaving copper end caps 20 connected to opposing ends thereof by hardsolder layer 22, the end caps being jointed to the circuit by conductingwires 24, the entire thermistor sensor structure being in a thermalresistance coating of an electrical insulator 26. A similar thermistorswitch assembly of the disc type is shown in FIG. 3 wherein the similarnumbers indicate similar portions of the thermistor assembly structure.

Standard metal oxide thermistors and separate fixed commercial resistorscan be utilized to achieve the abovedescribed results and to constructthe device described herein.

A high-temperature warning system of the type disclosed herein can bedesigned for any of a number of applications, the particular designthereof, i.e., the switching temperature, being determined by theparticular thermistor material utilized. Such systems can be used inrefrigerated areas wherein a rise of temperature above a predeterminedlevel would cause damage to the refrigerated goods, in mechanicalapplications wherein overheating of either a liquid such as automotiveoil or coolant or alternatively a specific area, i.e., within the brakeassembly, could be reported to the operator of the vehicle or equipmentwithin which the sensor is mounted. As should be clear to one skilled inthis art, the application of the circuit and system described herein arealmost endless and are limited only by the imagination of the user.

Although the invention has been described in detail with particularreference to particular embodiments thereof, it should be understoodthat alterations and modifications thereof may be made within the spiritand scope of the invention.

I claim:

1. A thermal warning system comprising a tungsten filament lamp, asensor thermistor device, and a direct current voltage source connectedwith said lamp and said device in series circuit relationship, wherein:

said lamp filament is of such a thermal mass relative to said devicethat its thennal time constant is longer than that of the thermistordevice;

the thermistor device comprises a body having a negative thermalcoefficient of resisiancei a pair of conductive wires, said wiresterminating at opposite faces of said body,

a hard solder connection forming a bond between the terminal end of asaid wire and its adjacent body face, there being one such connectionfor each said wire terminal, and

a conductive resistance film on said body between said connections ateach of two opposite faces of the body;

the thermal masses of the lamp and the thermistor device being suchthat, as thermistor resistance decreases with increasing temperature,the lamp filament heats and increases in resistance thereby reducingpower dissipation in the thermistor device and eventually switching thethermistor device back to its initial condition in which the resistanceof the thermistor device again decreases as its temperature increases,thereby providing cyclic heating and cooling and consequent alternatelybright and dull glowing of the lamp filament to indicate when thethermistor device is at or above a specified temperature.

2. A warning system as in claim 1, wherein the terminal ends of saidwires are provided with copper end caps in engagement thermally andelectrically with their respective hard solder connections.

1. A thermal warning system comprising a tungsten filament lamp, asensor thermistor device, and a direct current voltage source connectedwith said lamp and said device in series circuit relationship, wherein:said lamp filament is of such a thermal mass relative to said devicethat its thermal time constant is longer than that of the thermistordevice; the thermistor device comprises a body having a negative thermalcoefficient of resistance, a pair of conductive wires, said wiresterminating at opposite faces of said body, a hard solder connectionforming a bond between the terminal end of a said wire and its adjacentbody face, there being one such connection for each said wire terminal,and a conductive resistance film on said body between said connectionsat each of two opposite faces of the body; the thermal masses of thelamp and the thermistor device being such that, as thermistor resistancedecreases with increasing temperature, the lamp filament heats andincreases in resistance thereby reducing power dissipation in thethermistor device and eventually switching the thermistor device back toits initial condition in which the resistance of the thermistor deviceagain decreases as its temperature increases, thereby providing cyclicheating and cooling and consequent alternately bright and dull glowingof the lamp filament to indicate when the thermistor device is at orabove a specified temperature.
 2. A warning system as in claim 1,wherein the terminal ends of said wires are provided with copper endcaps in engagement thermally and electrically with their respective hardsolder connections.